In recent years, computer printer technology has evolved to a point where very high resolution images can be transferred to various media, including papers of different types. One particular type of printing involves the placement of small drops of a fluid ink onto a surface in response to a digital signal. Typically, the fluid ink is deposited or jetted onto the surface without physical contact between the printing device and the surface. In drop-on-demand inkjet printing, ink droplets are typically propelled from a nozzle by heat or by a pressure wave. Further information as to the basics of inkjet printing technology are further disclosed in various articles in several editions of the Hewlett-Packard Journal [Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994)], incorporated herein by reference.
In general, inkjet inks are either dye-based or pigment-based. Each type of ink offers different advantages when printing high-quality images. Dye-based inks typically use a liquid colorant that is usually water-based. The dye tends to be absorbed into the media surface, and turns the media the color of the dye. Dye-based inks are typically more chromatic and provide more highly saturated colors than pigmented inks. Because of their makeup, however, dye-based inks are usually not water-resistant. They also tend to be more affected by UV light, resulting in the color fading or changing over time.
Pigmented inks typically use a solid colorant to achieve color. With pigmented inks, solid particles remain on the surface of the print media. Once the water in the solution has evaporated, the particles will generally not go back into solution, and are therefore more water-resistant. In many cases, the line quality and accuracy of plots produced by pigment-based inks are superior to that produced by dye-based inks. In addition, pigmented inks are much more UV-resistant than dye-based inks, so that it typically takes a much longer time for noticeable fading of the printed media to occur.
Coated media used for inkjet printing of high-quality images typically have an ink-receptive overcoat, generally of a swellable polymer for absorbing the water-based inks and providing improved receptivity to pigmented inks. However, this special inkjet coated media is significantly more expensive than the coated media generally used in commercial offset printing of high-quality images using oil-based inks. Commercial offset coated media is significantly different from photo/glossy media specifically designed for use with inkjet aqueous-based inks. Typical commercial offset media have a less-porous surface comprised of a coating which requires more time for aqueous fluids to penetrate than standard porous paper. Additionally, offset coatings contain polymers that are more hydrophobic (e.g., styrene-butadiene based) than media coatings specifically designed for inkjet ink (e.g., water-soluble polymers such as polyvinyl alcohol). Thus, most inkjet inks typically produce poor results when used to print on commercial offset papers, showing long dry times, poor spreading characteristics, and poor adherence of pigment to the media coating.
As described in the above-referenced and commonly-owned co-pending U.S. application Ser. Nos. 09/675,043 and 09/702,169, improved pigmented inks have been designed having a binder resin which provides better adherence of the pigment to commercial offset media. Due to the complementary advantages in appearance and durability offered by pigmented inks and dye-based inks, it would be advantageous to be able to print a region of a desired color on commercial offset media using both pigmented inks and dye-based inks. However, dye-based inks typically stain the surface of the media, chemically changing it in such a manner that the binder resin is no longer as effective, thus undesirably degrading the adherence of the pigment to the media in the stained regions.
In a preferred embodiment, the present invention provides a method of inkjet printing using both pigmented inks and dye-based inks. A supply of pigmented ink of a certain color is provided, along with another supply of dye-based ink of the same color. A region of a media is printed with the color by depositing drops from the pigmented supply and drops from the dye-based supply on different subregions of the region.
The present invention may also be implemented as an inkjet printing system having a pigmented nozzle array and a dye-based nozzle array. The pigmented nozzle array controllably deposits drops of a pigmented ink of a certain color, while the dye-based nozzle array controllably deposits drops of a dye-based ink of the same color. A controller controls the depositing so as to place drops of the pigmented ink and drops of the dye-based ink on adjacent subregions of a region of a media.